Set to permanently underexpose?

I use an *istD and am delighted with it. However, shots containing white clothing are often burnt out and lose any texture in the fabric. This is particularly the case with a wedding dress.

I have been advised to set the camera with a permanent .5 or 1 stop underexposure, because it is easier to get detail from dark areas in Photoshop ... and impossible to do anything about areas that are burnt out. I can understand this logic.

Two questions:
1. Is the advice correct?
2. If yes, why is it necessary? Why cannot Pentax adjust the metering system to take account of bright white areas?

1) I think that the advice will work, so in that respect it could be correct.

2) Meters are set to integrate to a 18% grey reflectance (sometimes 10%) and that is all they do, so it can need some intelligent intervention to get the right result. CCDs have less dynamic range than film, so the chances are increased that highlights will be burnt out. I meter as I would for transparency film - another situation where we do not want burnt out highlights at any cost.

Multi-segment metering is much cleverer, but the trouble is we have to second-guess what the deisgner might have programmed the cameras systems to do. Give me simple spot or centre weighted metering any time.

Dare I mention that Fuji CCDs now have two photodiodes at every pixel point - one to record shadow detail, one to record highlights. If this works, it could be the way to go in CCD design.

Many thanks for the very helpful reply.
Malcolm

JR's advice is, as usual, spot on.

We have to remember that in any extreme situation the human brain is always going to know what is needed, and work out how to achieve it, much more effectively than a pre-programmed camera chip. If women wore all-white dresses all the time, no doubt cameras would be programmed to set the exposure appropriately.

George

I find that RAW mode always gives me about one extra stop for highlights and two stops under. It's saved the day many times for tricky lighting. Adobe Photoshop CS does a fine job, as does Capture One of handling the RAW files.

Do watch out for the fact that CCDs are linear, whereas film is not. A full half of the data values cover the top 1-stop. If you shoot in JPG, that's 128 values (8-bit = 256 values) in the top stop. If you underexpose too much, you will end up with seriously posterised images. Shooting RAW gives you 12-bits per pixel colour (4096 values), so there's less chance of posterisation if you underexpose.

Personally, I would tend to meter for the highlights (ie the dress), so that the metering tends to push it to 18% grey, giving a little extra for the highlights. Then shoot RAW and process and correct the grey point in Photoshop.

Matt

Again, many thanks for the advice. Much appreciated.
Malcolm